The following published patent documents are believed to represent the current state of the art:
U.S. Pat. Nos. 6,737,300; 6,828,175; 6,608,377; 6,103,552; 6,277,669; 6,492,201; 6,498,387; 6,727,576; 6,743,660 and 6,867,123; and
US Patent Application Publication Numbers: 2005/0260794; 2006/0017161; 2005/0046002; 2005/0012225; 2002/0109236; 2005/0056903; 2004/0222508; 2006/0115932 and 2006/0079019.